What would happen if I just bypassed this garden water check valve?

OK. I don't disagree. It's there for a reason.
I'm going to try to understand the thing and then repair it.
BTW, I can't tell if it's a 1" or a 1.5 inch opening
because it clearly has a "1" stamped on each opening
but it's just as clearly got a >1" pipe going in and out.
Here, for example, is a ruler next to the pipe and
a yellow circle around the 1.
LARGE:

SMALL:

Notice the pipe is actually about 1 3/8ths in diameter, so it's
neither 1 inch nor 1.5 inch.
Q: What size would you say this Wilkins Zurn 975XL actually fits?

You do realize that pipe is not really measured like it is called ?
That is what is called one inch pipe will actually measuer about 1.3 inches
outside diameter with a ruler.
http://www.pipemarkers.com/pipe-data.php
This is a pipe chart for sizes.

Jumping in late but just so you know...
Plumbers are odd folks compared to carpenters. A carpenter wants to know
how big a hole to drill for the pipe to fit through. The plumber measures
the inside diameter to know how much water he can get to flow through it.
I've had many a weekend warrior insist the 1/2" pipe he has in his hand is
really 3/4" and he is right as far as the size hole he needs to drill in the
joist but not as far as what should be on the label of the fittings he
needs. Let's face it, all the labels in my and every other store can not be
wrong so the way you look at it needs to be adjusted.

<snip>
I've also had instances of them wondering why a 1/2" socket doesn't fit the
ir 1/2" bolt. Best one was my neighbor who was building a two story additi
on. Stopped by and asked if it would rip several 1X4 in half to use as win
dow casings. I did as asked. He showed back up complaining they were too
narrow. I suspect he didn't believe my explanation that a 1x4 is only aobut
3 1/2" wide - bet he checked hat the lumber yard :).
Of course he was a 'country boy' but scoffed when I told him he needed to f
ence off that row of young trees he had just planted as his cows would eat
them if he didn't. He didn't and they did :).
Harry K

Not EXACTLY.. It depends on the thickness of the pipe. Schedule 40 pipe
is 1.049 inches inside, and sched 80 is .957 inches inside.
Pipe is the same outside diameter so it will all thread together for the
same size. Some pipe has thicker walls than others so the only way to go is
to make the inside diameter smaller.
Again look at the chart.
http://www.pipemarkers.com/pipe-data.php

Wrong place. Measure at the pipe into the check valve. Actually, it
doesn't much matter...get the size check valve necessary - I'd think 1 1/2"
would be very generous - and use pipe fittings as needed to marry it to the
pipe. Do you plan to keep the shutoff valves?

I have no idea. Best guess is for attaching pipes, no idea why you would
want to. I also have no idea why there are fore and aft valves.
I do see what appears to be a clean out fitting (two, actually). If it were
me I'd open them up and see if some gunk was keeping the valve open. If you
replace it, all you really need to know is the pipe sizes to which the new
check valve will attach. You will probably have to do some pipe fitting
too - nipple & coupling at least - as it is not likely a new one will fit
lengthwise properly. Cheapest check valves are PVC; brass is both better
and pricier.

I just got off the phone with Wilkins Zurn Technical Support
(aka Wilkins Water Works) at 1-855-663-9876
A new one is over seven hundred dollars.
So, while it may be easy to say to buy one, I would think
a home repair is in order here.
Luckily, they make a 'repair kit', with instructions:
http://www.zurn.com/Pages/ProductDetails.aspx?NodeKey77206
Wilkins Waterworks 975XL Repair Kit List Price: $55
Wilkins Waterworks 975XL Repair Kit List Price: $755

I called Technical Support again: 1-855-663-9876x2x4
and I detail the answers so the NEXT person with a
similar question can find the answers more easily.
Zurn Wilkins (they don't call it Wilkins Zurn) tech support
confirmed the (1) indicates it's a 1-inch fitting even though
the OD of the pipe going in is clearly (much) greater than
one inch in diameter.
They told me to look for an additional (1") on the body
and lo and behold, there it is (see this picture):

They only sell one repair kit which is the
RK34-975XL $43.20 (fits 3/4" and 1")
This repair kit comes with the thicker upper check valve
spring, the thinner lower check valve spring, 3 o-rings,
3 seals, etc., and as such is a "complete" repair kit.
They said that Furgeson Enterprises sells them:
http://www.ferguson.com
So, I think we have our solution!

We probably could second source the springs and o-rings in a second.
Or just refurbish the set.
I hope to get to this project this weekend so as to be able to report
back with pictures of the job.
Meanwhile, I'm waiting on parts for my other projects lined up!

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